The discovery of moriniafungin, a novel sordarin derivative produced by Morinia pestalozzioides

Sordarin - the antifungal antibiotic with a unique modus operandi

Fungal infections cause serious problems in many aspects of human life, in particular infections in immunocompromised patients represent serious problems. Current antifungal antibiotics target various metabolic pathways, predominantly the cell wall or cellular membrane. Numerous compounds are available to combat fungal infections, but their efficacy is far from being satisfactory and some of them display high toxicity. The emerging resistance represents a serious issue as well; hence, there is a considerable need for new anti-fungal compounds with lower toxicity and higher effectiveness. One of the unique antifungal antibiotics is sordarin, the only known compound that acts on the fungal translational machinery per se. Sordarin inhibits protein synthesis at the elongation step of the translational cycle, acting on eukaryotic translation elongation factor 2. In this review, we intend to deliver a robust scientific platform promoting the development of antifungal compounds, in particular focusing on the molecular action of sordarin.

A Quick Guide to Small-Molecule Inhibitors of Eukaryotic Protein Synthesis

Eukaryotic ribosome and cap-dependent translation are attractive targets in the antitumor, antiviral, anti-inflammatory, and antiparasitic therapies. Currently, a broad array of small-molecule drugs is known that specifically inhibit protein synthesis in eukaryotic cells. Many of them are well-studied ribosome-targeting antibiotics that block translocation, the peptidyl transferase center or the polypeptide exit tunnel, modulate the binding of translation machinery components to the ribosome, and induce miscoding, premature termination or stop codon readthrough. Such inhibitors are widely used as anticancer, anthelmintic and antifungal agents in medicine, as well as fungicides in agriculture. Chemicals that affect the accuracy of stop codon recognition are promising drugs for the nonsense suppression therapy of hereditary diseases and restoration of tumor suppressor function in cancer cells. Other compounds inhibit aminoacyl-tRNA synthetases, translation factors, and components of translation-associated signaling pathways, including mTOR kinase. Some of them have antidepressant, immunosuppressive and geroprotective properties. Translation inhibitors are also used in research for gene expression analysis by ribosome profiling, as well as in cell culture techniques. In this article, we review well-studied and less known inhibitors of eukaryotic protein synthesis (with the exception of mitochondrial and plastid translation) classified by their targets and briefly describe the action mechanisms of these compounds. We also present a continuously updated database (http://eupsic.belozersky.msu.ru/) that currently contains information on 370 inhibitors of eukaryotic protein synthesis.

Inhibitory Effect of Moriniafungin Produced by Setosphaeria rostrata F3736 on the Development of Rhizopus Rot

Rhizopus rot is a serious postharvest disease of various crops caused by Rhizopus spp. and controlled mainly by synthetic fungicides. We detected the antifungal activity of a culture extract of Setosphaeria rostrata F3736 against Rhizopus oryzae. The active ingredient was identified as moriniafungin, a known sordarin derivative, which showed minimum inhibitory concentrations of 1-8 μg/ml against Colletotrichum spp. and 0.03-0.13 μg/ml against Rhizopus spp. in vitro. Moriniafungin showed protective control efficacies against Rhizopus rot on apple and peach fruits. Treatment with 25 μg/ml moriniafungin delimited the lesion diameter significantly by 100% on R. oryzae-inoculated apple fruits compared with the non-treated control. Treatment with 0.04 μg/ml of moriniafungin reduced the lesion diameter significantly by 56.45%, and treatment with higher concentrations of 0.2-25 μg/ml reduced the lesion diameter by 70-90% on Rhizopus stolonifer var. stolonifer-inoculated peach fruit. These results suggest moriniafungin has potential as a control agent of postharvest diseases caused by Rhizopus spp.

Sordarin Diterpene Glycosides with an Unusual 1,3-Dioxolan-4-one Ring from the Zoanthid-Derived Fungus Curvularia hawaiiensis TA26-15

Six new sordarin tetracyclic diterpene glycosides, moriniafungins B-G (1-6), and a new sordaricin tetracyclic diterpene, sordaricin B (8), together with two known analogues, moriniafungin (7) and sordaricin (9), were isolated from the zoanthid-derived fungus Curvularia hawaiiensis TA26-15. The structures of the new compounds were elucidated by comprehensive analyses of spectroscopic data, including 1D and 2D NMR and MS data. Compounds 1-6 represent the first case of sordarins from marine-derived fungi possessing a sordarose with a spiro 1,3-dioxolan-4-one ring, which is rare in the nature. Compound 4 showed antifungal activity against Candida albicans ATCC10231 with an MIC value of 2.9 μM.

Extending the Metabolite Diversity of the Endophyte Dimorphosporicola tragani

Fungi are one of the most prolific sources of microbial secondary metabolites. The production of new metabolites can be achieved using multiple fermentation conditions and by adding small-molecule effectors, including epigenetic modifiers. In the framework of our Natural Product screening programme targeting the discovery of new antimicrobial compounds, we applied multiple fermentation conditions and adsorptive polymeric resins on a large collection of fungal endophytes, to increase and stimulate their fungal secondary metabolite production. During this work the endophytic fungus Dimorphosporicola tragani CF-090383 showed antimicrobial activity only when grown in presence of adsorptive polymeric resins. In addition, seven epigenetic modifiers were added to fermentations of this endophytic fungus, in an attempt to activate its cryptic pathways as well as to analyse the metabolites produced under these conditions. D. tragani was seen to produce three different mycotoxin dendrodolides when the epigenetic modifiers 5-azacytidine and valproic acid were added to the fermentations, and these compounds were further characterized. However, the fungus produced the fatty acid synthesis inhibitor cerulenin, a molecule not previously described to be produced by this fungal species, only when cultivated in presence of the XAD-16 resin. We have found that the addition of XAD-16 resin resulted in four-fold higher titers in the production of cerulenin when compared to the best production conditions described in literature for the original fungal producer strain, Cephalosporium caerulens KF-140 (=Sarocladium oryzae), in a zeolite-based fermentation, used as an ammonium ion-trapping agent. The production of cerulenin by this strain of D. tragani, represents an alternative source for the improved production of cerulenin with better yields.

Fungal endophytes from arid areas of Andalusia: high potential sources for antifungal and antitumoral agents

Native plant communities from arid areas present distinctive characteristics to survive in extreme conditions. The large number of poorly studied endemic plants represents a unique potential source for the discovery of novel fungal symbionts as well as host-specific endophytes not yet described. The addition of adsorptive polymeric resins in fungal fermentations has been seen to promote the production of new secondary metabolites and is a tool used consistently to generate new compounds with potential biological activities. A total of 349 fungal strains isolated from 63 selected plant species from arid ecosystems located in the southeast of the Iberian Peninsula, were characterized morphologically as well as based on their ITS/28S ribosomal gene sequences. The fungal community isolated was distributed among 19 orders including Basidiomycetes and Ascomycetes, being Pleosporales the most abundant order. In total, 107 different genera were identified being Neocamarosporium the genus most frequently isolated from these plants, followed by Preussia and Alternaria. Strains were grown in four different media in presence and absence of selected resins to promote chemical diversity generation of new secondary metabolites. Fermentation extracts were evaluated, looking for new antifungal activities against plant and human fungal pathogens, as well as, cytotoxic activities against the human liver cancer cell line HepG2. From the 349 isolates tested, 126 (36%) exhibited significant bioactivities including 58 strains with exclusive antifungal properties and 33 strains with exclusive activity against the HepG2 hepatocellular carcinoma cell line. After LCMS analysis, 68 known bioactive secondary metabolites could be identified as produced by 96 strains, and 12 likely unknown compounds were found in a subset of 14 fungal endophytes. The chemical profiles of the differential expression of induced activities were compared. As proof of concept, ten active secondary metabolites only produced in the presence of resins were purified and identified. The structures of three of these compounds were new and herein are elucidated.

Structure-based designing of sordarin derivative as potential fungicide with pan-fungal activity

Fungal infections have become a significant problem for immunosuppressed patients. Sordarin, a promising fungicidal agent, inhibits fungal protein synthesis by impairing elongation factor-2 (eEF2) function. Intriguingly, despite high sequence similarity among eEF2s from different species, sordarin has been shown to inhibit translation specifically in certain fungi while unable to do so in some other fungal species (e.g. Candida parapsilosis and Candida lusitaniae). The sordarin binding site on eEF2 as well as its mechanism of action is known. In a previous study, we have detailed the interactions between sordarin and eEF2 cavities from different fungal species at the molecular level and predicted the probable cause of sordarin sensitivity. Guided by our previous analysis, we aimed for computer-aided designing of sordarin derivatives as potential fungicidal agents that still remain ineffective against human eEF2. We have performed structural knowledge-based designing of several sordarin derivatives and evaluated predicted interactions of those derivatives with the sordarin-binding cavities of different eEF2s, against which sordarin shows no inhibitory action. Our analyses identify an amino-pyrrole derivative as a good template for further designing of promising broad-spectrum antifungal agents. The drug likeness and ADMET prediction on this derivative also supports its suitability as a drug candidate.

FR290581, a novel sordarin derivative: synthesis and antifungal activity

Sordarin is a unique natural product antifungal agent that is an inhibitor of elongation factor 2. To improve biological activity, we synthesized various compounds by novel modification of the aglycone, sordaricin. As a result, we have discovered the novel sordarin derivative FR290581. This compound exhibited superior activity and a good pharmacokinetic profile, and also displayed good in vivo activity against Candida albicans.

Biodiversity, chemical diversity and drug discovery

Drugs developed from microbial natural products are in the fundaments of modern pharmaceutical companies. Despite decades of research, all evidences suggest that there must remain many interesting natural molecules with potential therapeutic application yet to be discovered. Any efforts to successfully exploit the chemical diversity of microbial secondary metabolites need to rely heavily on a good understanding of microbial diversity, being the working hypothesis that maximizing biological diversity is the key strategy to maximizing chemical diversity. This chapter presents an overview of diverse topics related with this basic principle, always in relation with the discovery of novel secondary metabolites. The types of microorganisms more frequently used for natural products discovery are briefly reviewed, as well as the differences between terrestrial and marine habitats as sources of bioactive secondary metabolite producers. The concepts about microbial diversity as applied to prokaryotes have evolved in the last years, but recent data suggest the existence of true biogeographic patterns of bacterial diversity, which are also discussed. Special attention is dedicated to the existing strategies to exploit the microbial diversity that is not easy to tackle by conventional approaches. This refers explicitly to the current attempts to isolate and cultivate the previously uncultured bacteria, including the application of high throughput techniques. Likewise, the advances of microbial molecular biology has allowed the development of metagenomic approaches, i.e., the expression of biosynthetic pathways directly obtained from environmental DNA and cloned in a suitable host, as another way of accessing microbial genetic resources. Also, approaches relying on the genomics of metabolite producers are reviewed.

The value of natural products to future pharmaceutical discovery

Natural products have provided considerable value to the pharmaceutical industry over the past half century. In particular, the therapeutic areas of infectious diseases and oncology have benefited from numerous drug classes derived from natural product sources. Unfortunately, pharmaceutical companies have significantly decreased activities in natural product discovery during the past several years. Biotechnology companies working in the fields of combinatorial biosynthesis, genetic engineering and metagenomic approaches to identify novel natural product lead molecules have had limited success. Despite what appears to be a slow death of natural product discovery research, many new and interesting molecules with biological activity have been published in the past few years. If natural product materials continue to be tested for desirable therapeutic activities, we believe that significant progress in identifying new antibiotics, oncology therapeutics and other useful medicines will be made.

Sordarin derivatives induce a novel conformation of the yeast ribosome translocation factor eEF2

The sordarins are fungal specific inhibitors of the translation factor eEF2, which catalyzes the translocation of tRNA and mRNA after peptide bond formation. We have determined the crystal structures of eEF2 in complex with two novel sordarin derivatives. In both structures, the three domains of eEF2 that form the ligand-binding pocket are oriented in a different manner relative to the rest of eEF2 compared with our previous structure of eEF2 in complex with the parent natural product sordarin. Yeast eEF2 is also shown to bind adenylic nucleotides, which can be displaced by sordarin, suggesting that ADP or ATP also bind to the three C-terminal domains of eEF2. Fusidic acid is a universal inhibitor of translation that targets EF-G or eEF2 and is widely used as an antibiotic against Gram-positive bacteria. Based on mutations conferring resistance to fusidic acid, cryo-EM reconstructions, and x-ray structures of eEF2, EF-G, and an EF-G homolog, we suggest that the conformation of EF-G stalled on the 70 S ribosome by fusidic acid is similar to that of eEF2 trapped on the 80 S ribosome by sordarin.